I've had a read of the documentation but it seems aimed at more experienced engineers with familiarity with electronic simulation. I'm trying to refresh the basic electronics I learned at University, and this is my first time with electronic simulation.

@DuncanBayne the 'why' is because the circuit doesn't know what your +9v symbol means: it's just another name for the net. You have to create the voltage inserting a voltage source.
–
clabacchio♦Apr 24 '12 at 12:05